The release of neurotransmitters from presynaptic nerve terminals by the fusion of transmitter-filled secretory vesicles with the nerve terminal membrane is critical for the transfer of information throughout the nervous system. This release is gated by the influx of calcium ions via voltage-gated ion channels and the control of channel availability is a major mechanism in the control of synaptic strength, and hence, synaptic pathways. We have examined presynaptic N-type calcium channels on a presynaptic nerve terminal using the patch clamp technique to record ion currents directly from the calyx-type terminal in the cholinergic synapse in the chick ciliary ganglion. With this preparation we have tested for calcium channel regulation via G-proteins by injecting a non-hydrolysable analogue of GTP, GTPgammaS, into the terminal while activating the channels with a depolarizing potential. We report two main findings. First, presynaptic calcium channels exhibit a voltage-sensitive down regulation as observed previously in neuronal somata on activation of G proteins. Second, this down- regulation could be eliminated by prior treatment with botulinum toxin C1, a toxin that selectively cleaves the transmitter release-associated protein syntaxin. These findings support the hypothesis that calcium channels in nerve terminals are subject to modulation via pathways that culminate in the activation of G proteins. Furthermore, they provide the first evidence that in the presynaptic nerve terminal, the G protein-based mechanism of calcium channel modulation is directly associated with the mechanism whereby transmitter is released from secretory vesicles.